Exhaust gas recirculation (EGR) is a technology used with internal combustion engines, and in the case of gasoline engines, is primarily used to improve engine efficiency. EGR works by recirculating a portion of an engine's exhaust gas back to the engine cylinders. The recirculated exhaust, added to the fuel, oxygen, and combustion products, increases the specific heat capacity of the cylinder contents, which lowers the adiabatic flame temperature. In a gasoline engine, the recirculated exhaust displaces the amount of combustible matter in the cylinder.
For a gasoline engine, a properly operating cooled EGR system improves the knock tolerance of the engine, which allows improved combustion phasing and increased compression ratio. The combined effect of the improved phasing and compression ratio improves efficiency and reduces fuel consumption.